The present invention relates to an image converter and in particular to an apparatus for conducting processes of cutting out a part of a distorted circular image photographed by use of a fisheye lens and converting it into a planar regular image.
A fisheye lens can be used to obtain hemispherical circular images in all directions without using a mechanical moving mechanism. Therefore, a fisheye lens has been extensively used in photographing scenic pictures and others aiming at eccentric effect. However, images photographed by use of a fisheye lens are distorted in circular images, and the images may be used for artistic pictures, as they are, but not suitable for general photographic uses.
Thus, there has been proposed an apparatus for conducting process of converting a distorted circular image photographed by use of a fisheye lens into a planar regular image with less distortion. For example, Japanese Patents No. 3012142 and No. 3051173 have disclosed technologies by which a computer is used to convert a portion of a distorted circular image to a planar regular image in real time. When this conversion technique is utilized, it is possible to convert a dynamic image made up of distorted circular images photographed by use of a fisheye lens to a dynamic image made up of planar regular images and observe it in real time. Application to a monitoring system and others having a 180-degree field angle is expected.
On the other hand, in Japanese Patent No. 3025255, there has been disclosed a technique for providing a panoramic display in the horizontal direction by converting a distorted circular image photographed by use of a fisheye lens into an image developed on a cylindrical face. Further, in Japanese Patent No. 3126955, there has been disclosed a technique for relieving the load of hardware by replacing division and other function operations necessary on conversion of a distorted circular image to a planar regular image with a process to refer to a lookup table.
In recent years, there has been widely used a fisheye lens monitoring system in which a security camera equipped with a fisheye lens is installed on ceilings and walls of a building to display an image photographed by use of the security camera on a monitoring device. In this application, it is required to obtain a planar regular image from which any distortion is removed as much as possible due to a necessity for recognizing the face of a person and features of clothing on a monitoring screen. However, it is difficult to obtain such a distortion-free planar regular image that can satisfy the above demand by using image converters conventionally proposed.
For example, use of the techniques disclosed in the above-described Japanese Patents No. 3012142 and No. 3051173 enables to obtain a rectangular planar regular image by cutting out a part desired by a user from a distorted circular image photographed by use of a fisheye lens. However, the thus obtained planar regular image is distorted and inadequate for an application to confirm detailed features of a person. In particular, since the thus obtained rectangular planar regular image is greatly distorted in particular at an external part (in the vicinity of a contour), it is impossible to obtain a smooth panning image in a case where a cut-out portion is gradually moved to conduct a horizontal panning.
On the other hand, the technique disclosed in the above-described Japanese Patent No. 3025255 provides a panoramic image longer in the horizontal direction. Therefore, a part of the image is cut out to be displayed on a monitoring screen, thus making it possible to obtain a smooth panning image even when a horizontal panning is conducted. However, a panoramic image obtained by this technique is distorted greatly in the perpendicular direction although distorted to a small extent in the horizontal direction, and restriction is imposed on a region at which an image can be cut out. More specifically, even if a semi-sphere region is photographed, a planar regular image which is cut out from the vicinity of the top is greatly distorted. Therefore, it is actually impossible to cut out the vicinity of the top. As described above, if restriction is imposed on a part of an image that can be cut out, it is unfavorable because a blind spot is caused when the technique is applied to a security camera.
Therefore, an object of the present invention is to provide an image converter which is capable of cutting out any desired part from a distorted circular image photographed by use of a fisheye lens to convert it into a planar regular image with less distortion.